Mass spectrometers have been used to analyse a wide range of materials, including organic substances such as pharmaceutical compounds, environmental compounds and biomolecules. They are particularly useful, for example, for DNA and protein sequencing. In such applications, there is an ever increasing desire for faster speed of analysis of sample ions by the mass spectrometer while still producing accurate measurements of the mass/charge ratios of the ions in question.
Some steps towards increasing the speed of analysis of mass spectrometers have been taken by increasing the number of inlets to the mass spectrometer. For example, in Analytical Chemistry 2000, 72, pages 20-24, L. Jiang and M. Moini describes a dual- or quad-orifice mass spectrometer which receives sample ions from two or four electrospray ionisation sources respectively. In this way, several streams of sample ions can be analysed simultaneously and furthermore a stream of reference ions can be introduced into the mass spectrometer at the same time as the stream of sample ions, enabling more accurate readings.
An alternative arrangement is shown in EP-A-0,966,022. A mass spectrometer is shown having a single sampling orifice for receiving ions and a plurality of sample ion sources. The sampling orifice is connected to a sample selector with at least one aperture. Each ion source generates a jet of ions directed towards the sample selector which may rotate to align an aperture with any one of the jets. In this manner any one of a number of different jets of sample ions to be analysed may enter the sample selector, pass through the sampling orifice and into the mass spectrometer.
U.S. Pat. No. 6,066,848 provides, in a first embodiment, an ion source having a plurality of sample ionisers and in which a rotatable disc having a small hole is provided adjacent the inlet orifice of a mass spectrometer. Rotation of the disc allows a selected one of the streams of sample ions to enter the analyser. In a second embodiment, the disc is stationary and is provided with number of shutter valves which may be individually opened or closed to permit one of the streams of sample ions to enter the analyser.
The prior art mass spectrometers described above all suffer from various drawbacks. For example, in the case of the arrangement described by Jiang and Moini, providing a plurality of orifices leading into the mass spectrometer requires extra pumps to maintain a suitably low pressure within the mass spectrometer, especially with the quad-nozzle arrangement. In the case of the devices shown in EP-A-0,966,022 and the first embodiment of U.S. Pat. No. 6,066,848, the rotating sample selectors are both cumbersome, slow to react, and also difficult to engineer reliably. Finally, none of these devices are particularly fast in switching from one sample stream to the next.
It is an object of the present invention to address these problems with the prior art.